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Colloidal Structure-directing-agent-free Hydroxy-sodalite Nanocrystals
Jianfeng Yao and Huanting Wang*
Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia,
Tel: +61 3 9905 3449, Fax: +61 3 9905 5686, E-mail: [email protected]
ABSTRACT
Our previous work showed that colloidal structure-
directing-agent-free hydroxy-sodalite nanocrystals could be
synthesized by the direct transformation of silicalite
nanocrystals. The structure directing agent TPAOH
molecules trapped in the dried silicalite nanocrystals proved
to play an important role in confining the structure
transformation. In the presence of TPAOH, silicalite
nanocrystals reacted with alkaline NaO-Al2O3 aqueous
solution and formed hydroxy-sodalite nanocrystals without
substantial change in the sizes and morphologies. In this
presentation, different colloidal silica sources were used to
examine the effect of the crystalline structure of starting
nanoparticles. The results indicated that sodalite crystals
and a mixture of zeolite A and sodalite crystals were
produced, and their sizes ranged from less than 100 nm to a
couple of microns when amorphous colloidal silica
nanoparticles were used. This again suggests that a
combination of silicalite nanocrystals and TPAOH is
advantageous for formation of hydroxy-sodalite
nanocrystals.
Keywords: silicalite, sodalite, nanocrystals, transformation,
structure directing agent
1 INTRODUCTION
There has been continuously growing interest in the
synthesis and application of zeolite nanocrystals since
colloidal zeolite suspension was first reported in the early
1970s. [1] Owing to the success of the synthesis of zeolite
nanocrystals, it has become more convenient for studying
the mechanisms of zeolite crystallization; the improved
performance of zeolites has been seen in their traditional
uses such as in catalysis, ion exchange and adsorption , and
more importantly zeolite nanocrystals have brought about
some new applications including low-k dielectrics in
microelectronics, optoelectronics, chemical sensing and
nanostructured membranes in fuel cells and separation
processes.
To produce colloidal zeolite nanocrystals, a large
amount of organic structure directing agent (SDA) is
required in most syntheses, and the use of SDA makes
zeolite crystallization controllable. Recently, SDA-free
syntheses of zeolite nanocrystals have been developed
through the use of the space-confinement additives such as
carbon black, starch, gelling polymer and even carbon
nanotubes. However, a novel method for rational synthesis
of the zeolite nanocrystals with narrow size distribution is
still highly demanding. [1]
Amongst various types of zeolites, sodalite is very
attractive as functional material for a wide range of
applications such as optical materials, waste management,
hydrogen storage and hydrogen separation. Colloidal
hydroxy-sodalite nanocrystal sols were synthesized by
homogeneous nucleation in the presence of
tetramethylammonium hydroxide (TPAOH). [2] By solid–
solid transformation of pillared clay SDA-free, sodalite
particles (500 nm) composed of small crystallites were
synthesized. [3-5] Very recently, we have reported our
findings that colloidal SDA-free hydroxy-sodalite
nanocrsystals can be obtained by the transformation of
silicalite-1 nanocrystals. [6] Silicalite nanocrystals were
employed as the silica source for hydroxy-sodalite synthesis
since synthesis of silicalite had been well established. After
the incorporation of the alkaline solution and the
transformation of crystal structure, the resulting hydroxy-
sodalite nanocrystals possessed the similar sizes and
morphologies as the original silicalite nanocrystals. In this
presentation, we will further the use of different silica
sources in the synthesis of sodalite and discuss the effect of
silica source on sodalite growth.
2 EXPERIMENTAL
2.1 Sample preparation
Two SiO2 sols, i.e., Ludox HS-30 (Sigma Aldrich), and
Snowtex 20L (20wt%, 40-50 nm, Nissan Chemicals) were
used as received. TEOS (tetraethyl orthosilicate, Sigma-
Aldrich) derived sol was prepared by mixing 10 g of TEOS,
7.1 g of deionized water and 1 g of 4 M HCl, followed by
stirring at room temperature for 3h. TPAOH
(tetrapropylammonium hydroxide) solution (1 M, Sigma
Aldrich) was added under stirring into Ludox HS-30 sol,
TEOS derived sol and Snowtex 20L sol, respectively. The
silica sources with TPAOH were obtained by directly
drying the three mixtures with a molar composition of 1
TPAOH: 4.8 SiO2: 44 H2O at 80-90 C. As a comparison,
another batches of three silica sols were prepared and the
dried under the same conditions without adding TPAOH.
439NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-6-5 Vol. 1, 2006
A colloidal silicalite nanocrystal solution was
synthesized as follows. [7] A clear synthesis solution was
prepared by dropwise addition of 1 M TPAOH into TEOS
under stirring, and stirring continued at room temperature
for 3 h. The final solution composition was 1 TPAOH: 4.8
SiO2: 44 H2O. After crystallization at 80 oC for 3-4 days,
colloidal silicalite suspension was dried at 80-90 C as
silica source for synthesis of sodalite nanocrystals.
The alkaline solution with a molar composition of 6.07
Na2O: 1 Al2O3: 66.0 H2O was prepared by mixing 20 g of
sodium hydroxide (Merck), 9.2 g of sodium aluminum
(Sigma-Aldrich) and 60 g of deionized water at room
temperature for 1-2 h. Different silica sources, with an
equivalent weight of 0.72 g silica, was added in 11 g of the
alkaline solution, and aged at room temperature for 4 h. The
crystallization was conducted at 80 oC for 3 h. The samples
obtained were cooled to room temperature, and the zeolite
crystals were collected by repeating three cycles of washing
with deionized water and centrifugation, followed by
drying at 90-100 oC overnight.
2.2 Characterization
Scanning electron microscopy (SEM) images were
taken with a JSM-6300F microscope (JEOL), and
transmission electron microscopy (TEM) images were
taken with a Philips CM120 microscope. X-ray diffraction
(XRD) patterns were collected with a Philips PW1140/90
diffractometer using Cu K radiation. Thermogravimetric
analysis (TGA) (Perkin Elmer, Pyris 1 TGA) was
conducted in air at a heating rate of 5 oC/min up to 700 oC.
3 RESULTS AND DISCUSSION
3.1 Dried silica sols as silica sources
Fig. 1 shows SEM images of the samples from three dried
silica samples without adding TPAOH. Microsized crystals
were formed from Ludox HS-30 and TEOS derived silicas,
(Fig. 1a,b) whereas zeolite nanocrystals (50-60 nm) were
produced from Snowtex 20L silica (Fig. 1c). XRD patterns
shown in Fig.2 revealed that all of three samples are zeolite
A mixed with a small amount of sodalite. This clearly
indicates that the silica particle size has a significant
influence on the zeolite crystal size. During aging and
hydrothermal treatment, Ludox HS-30 and TEOS derived
silicas can be easily dissolved to form amorphous NaO-
SiO2-Al2O3-H2O gels, in which solution-mediated
nucleation and crystallization facilitates zeolite growth.
Therefore, large crystals tend to grow from dried Ludox
HS-30 and TEOS derived sols with small silica particles.
By contrast, it took a much longer time to dissolve Snowtex
20L silica nanoparticles (40-50 nm in size) in the alkaline
solution. Highly likely, silica dissolution is not complete
before zeolite crystallization is carried out at hydrothermal
conditions. A large number of nuclei are produced from
localized NaO-SiO2-Al2O3-H2O gels leading to small
crystal sizes.
Effect of hydrothermal time was also examined in this
study. A mixture of zeolite A and sodalite was obtained
from all amorphous silica sources after 2 h of hydrothermal
reaction whereas pure sodalite was produced when was
extended to 3 h. [6] When hydrothermal time was extended
to 16 h for dried Snowtex 20L silica sol, pure sodalite
crystals were synthesized as shown in Fig. 3. However, the
crystal sizes grew up to around 200 nm. This result suggests
that the crystal structure evolves from zeolite A to sodalite
under present synthesis conditions.
Figure 1 SEM images of zeolites prepared from dried silica
sols without TPAOH. (a) Ludox HS-30 sol, (b) TEOS
derived sol, and (c) Snowtex 20L sol.
Figure 2 XRD patterns of zeolites prepared from dried
silica sols without TPAOH. (a) Ludox HS-30 sol, (b) TEOS
derived sol, and (c) Snowtex 20L sol.
440 NSTI-Nanotech 2006, www.nsti.org, ISBN 0-9767985-6-5 Vol. 1, 2006
3.2 Dried silica sols with TPAOH as silica
sources
In the presence of TPAOH molecules in dried silica sols,
zeolite nanocrystals were produced from all three silica
sources (Fig.4). It is obvious that zeolite crystal sizes grown
with TPAOH molecules around are much smaller than
those prepared from silica sol without TPAOH involved
(Fig. 1). Crystal structure of zeolite nanocrystals was
identified by XRD (Fig.5) to be a mixture of zeolite A and
sodalite. However, as compared with those zeolites
produced without using TPAOH (Fig.2), the proportion of
sodalite phase in all samples is substantially larger. As a
result, TPAOH molecules can not only act as a barrier
preventing big crystal formation, but also promote the silica
transformation into sodalite.
Figure 3 XRD pattern of sodalite prepared from directly
dried Snowtex 20L silica sol (40-50 nm in size) and
hydrothermal for 16 h.
3.3 Silicalite nanocrystals as silica source
When silicalite nanocrstals dried from colloidal
suspension were used as silica source, pure sodalite
nanocrystals were produced. Interestingly, sodalite
nanocrystals exhibit similar morphologies. The average
crystal size somewhat decreases from about 80 nm for
silicalite to 60 nm after phase transformation of silicalite
into sodalite (Fig. 6). The crystalline nature of silicalite
seems to facilitate sodalite formation, and offer high
resistance to dissolution in alkaline solution. Therefore a
combination of silicalite nanocrystals and TPAOH
molecules forms a unique way to synthesizing sodalite
nanocrystals.
Figure 4 SEM images of zeolites prepared from dried
Ludox HS-30 sol-TPAOH (a), dried TEOS derived sol-
TPAOH (b) and dried Snowtex 20L sol-TPAOH (c).
Figure 5 XRD patterns of zeolites prepared from dried
Ludox HS-30 sol-TPAOH (a), dried TEOS derived sol-
TPAOH (b), and dried Snowtex 20L silica sol-TPAOH (c).
Figure 6 TEM images of silicalite (a) and hydroxy-sodalite
(SOD-D) (b) from directly dried silicalite
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4 CONCLUSIONS
Colloidal SDA-free hydroxy-sodalite nanocrystals were
prepared by the transformation of silicalite nanocrystals. A
mixture of zeolite A and sodalite was grown from
amorphous silica sources. The presence of TPAOH
molecules significantly reduced crystal sizes of zeolites
synthesized from amorphous silicas. It can be concluded
that silicalite is preferred for the formation of sodalite
nanocrystals, and TPAOH molecules serve as a temporary
barrier and effectively confine the zeolite transformation.
Work is in progress to synthesize sodalite nanocrysals from
amorphous silicas. It is likely other types of zeolite
nanocrystals such as A, X and Y would be synthesized from
both silicalite and amorphous silica by optimizing synthetic
conditions.
5 ACKNOWLEDGEMENTS
This work was financially supported by the Australian
Research Council (Discovery Project DP0559724) and
Monash University. The authors gratefully acknowledge the
staff at the Electron Microscopy and Microanalysis Facility
of Monash University for their technical assistance with
sample characterizations. We thank Nissan Chemicals for
providing us with Snowtex silica sols. H.W. would like to
thank the Australian Research Council for the QEII
Fellowship.
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